Phosphorus-Bridged ansa-Metallocene Complexes of Titanium, Zirconium, and Hafnium:  The Syntheses and Structures of [PhP(C₅Me₄)₂]MX₂ and [Ph(E)P(C₅Me₄)₂]MX₂ (E = O, S, Se) Derivatives

A series of phosphorus-bridged ansa -metallocene complexes of titanium, zirconium, and hafnium, [PhP(C 5 Me 4 ) 2 ]MX 2 and [Ph(E)P(C 5 Me 4 ) 2 ]MX 2 (X = Cl, Me, CO, (Se 3 ) 0.5, (Te 3 ) 0.5; E = O, S, Se), has been synthesized. Structural characterization by X-ray diffraction indicates that, in comparison to their non- ansa counterparts (C 5 Me 5 ) 2 MX 2, the cyclopentadienyl groups in phosphorus-bridged complexes are displaced from symmetric η 5 -coordination toward η 3 -coordination. Such η 3,η 3 -coordination creates more electrophilic metal centers than those in their permethylcyclopentadienyl counterparts, as judged by the ν(CO) stretching frequencies of the zirconium dicarbonyl complexes Cp* 2 Zr(CO) 2 (1946 and 1853 cm - 1 ) and [PhP(C 5 Me 4 ) 2 ]Zr(CO) 2 (1959 and 1874 cm - 1 ).